It has conventionally been known that a polymethylsilsesquioxane powder is obtained by hydrolysis condensation of a trifunctional silane such as methyltrichlorosilane. For example, Belgian Patent No. 572,412 discloses a process for producing solid polymethylsilsesquioxane powder by hydrolyzing methyltrichlorosilane and water in an atomized state while adding dropwise the methyltrichlorosilane to a large amount of water under stirring. Such a process, however, is disadvantageous in that the polymethylsilsesquioxane powder produced contains a relatively large amount of chlorine atoms which have been produced as a by-product. In order to overcome this problem, JP-A-54-72300 discloses a method in which a methyltrialkoxysilane and/or a partial hydrolyzate thereof is hydrolyzed and condensed in an aqueous solution containing an alkaline earth metal hydroxide or an alkaline metal carbonate. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
Although the above method can eliminate the problem of the inclusion of chlorine atoms in the product, it brings about a new problem that the polymethylsilsesquioxane powder produced contains a relatively large amount of an alkaline earth metal or an alkali metal. JP-A-60-13813 discloses a process for producing polymethylsilsesquioxane powder free from the above-described problems and also having excellent fluidity by hydrolyzing and condensing a methyltrialkoxysilane and/or a partial hydrolyzate thereof in an aqueous solution of ammonia and/or amine. Further, JP-A-63-77940 discloses almost true spherical polymethylsilsesquioxane powder. Such a polymethylsilsesquioxane powder obtained by the above conventional techniques is useful as an additive for not only improving the durability and lubricating properties of rubbers or plastics but also imparting water repellency thereto.
However, rubbers, plastics or the like to which the polymethylsilsesquioxane powder obtained by the above prior art method has been added do not always show sufficient water repellency for some uses. This is because a slight quantity of silanol groups exists on the surface of the polymethylsilsesquioxane powder obtained by the prior art method, which leads to the inadequate water repellency of the rubbers or plastics.
One of the present inventors has disclosed that the electrostatically charging property can be imparted to the spherical polymethylsilsesquioxane powder by treating the powder with a compound having at least two functional groups such as alkoxy group in JP-A-63-101855. However, such a polymethylsilsesquioxane powder does not also have sufficiently improved water repellency.